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Eyelid development, fusion and subsequent reopening in the
mouse
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Findlater, GS, McDougall, RD & Kaufman, MH 1993, 'Eyelid development, fusion and subsequent
reopening in the mouse' Journal of Anatomy, vol 183 ( Pt 1), pp. 121-9.
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Download date: 19. Jun. 2017
J. Anat. (1993), 183, pp. 121-129, with 20 figures
121
Printed in Great Britain
Eyelid development, fusion and subsequent reopening in the
mouse
G. S. FINDLATER, R. D. McDOUGALL AND M. H. KAUFMAN
Department of Anatomy, University Medical School, Edinburgh, UK
(Accepted 29 March 1993)
ABSTRACT
The process of eyelid development was studied in the mouse. The critical events occur between about 15.5 d
postcoitum (p.c.) and 12 d after birth, and were studied by conventional histology and by scanning electron
microscopy. At about 15.5 d p.c. the cornea of the eye is clearly visible with the primitive eyelids being
represented by protruding ridges of epithelium at its periphery. Over the next 24 h, eyelid development
proceeds to the stage when the cornea is completely covered by the fused eyelids. Periderm cells stream in to
fill the gap between the developing eyelids. Their proliferative activity is such that they produce a cellular
excrescence on the outer surface of the line of fusion of the eyelids. This excrescence had almost disappeared
by about 17.5 d p.c. Keratinisation is first evident at this stage on the surface of the eyelids and passes
continuously from one eyelid to the other. Evidence of epidermal differentiation is more clearly seen in the
newborn, where a distinctive stratum granulosum now occupies about one third of its entire thickness.
Within the subjacent dermis, hair follicles are differentiating. By about 5 d after birth, a thick layer of
keratin extends without interruption across the junctional region. While a noticeable surface indentation
overlies the latter, a similar depression is only seen on the conjunctival surface by about 10 d after birth.
Keratinisation is also observed to extend in from the epidermal surface to involve the entire region between
the 2 eyelids at about this time. Numerous mature hair follicles are also present within the dermis at this
stage, as well as differentiated muscle fibres of orbicularis oculi. By about 12 d after birth, squames of
keratin are located between the 2 eyelids, and eyelid opening occurs rapidly thereafter. While the sequence
of events in the mouse is similar to that described in the human, the histological events associated with the
closure and subsequent reopening of the eyelids have not previously been described in detail in any species.
INTRODUCTION
Eyelid development, fusion and subsequent reopening
were first described in the rat by Addison & How
(1921). They found that in a 17 d fetus the primitive
eyelids had the form of protruding ridges which were
just visible around the margins of the developing eye.
However, subsequent eyelid development was apparently so rapid that by d 18 of gestation the eyes
were completely covered by the fused eyelids. At birth,
gestational age d 22, the eyelids were tightly closed
and remained so until 14 d after birth, when eyelid
opening finally occurred.
Eyelid development, closure and subsequent reopening is a feature common to all mammals. Whether
a particular animal species is born with its eyelids
open or closed is believed to be determined by the
stage of development of the animal at the time of
birth. In human embryos, for example, the eyelids
start to form at about 40-45 d of prenatal life with
complete fusion occurring approximately 15 d later
(Pearson, 1980). The eyelids remain closed thereafter
until the seventh month of fetal life when they reopen
(Hamilton & Mossman, 1972).
In their work on the prenatal development of the
mouse eye, Pei & Rhodin (1970) made passing
observations on the development of the eyelids. They
found that formation of the eyelids in the mouse
began at approximately 13-14 d of gestation with
fusion occurring sometime during the subsequent 2 d.
They did not, however, observe events after birth and
therefore did not view the process of eyelid dys-
Correspondence to Dr G. S. Findlater, Department of Anatomy, University Medical School, Teviot Place, Edinburgh EH8 9AG, UK.
122
G. S. Findlater, R. D. McDougall and M. H. Kaufman
Figures 1 to 7 are scanning electron micrographs from the period gestational age 15.5 to 16.5 d p.c.
Fig. 1. Early in this period the cornea (C) can be clearly seen within the margins of the developing eyelids (arrowheads) x 100.
123
Eyelid closure and reopening in the mouse
junction. Harris & McLeod (1982) carried out a
scanning electron microscopic study on eyelid growth
and fusion in embryonic/fetal mice. They confirmed
many of the observations of Addison & How (1921)
and of Pei & Rhodin (1970), but did not follow the
process through to the stage of eyelid reopening.
Whereas other studies have used light microscopy
to investigate eyelid development in the rat (Addison
& How, 1921) and mouse (Pei & Rhodin 1970), and
the scanning electron microscope to observe the
external features of eyelid development in the mouse
(Harris & McLeod, 1982), and then only as far as
eyelid fusion, this study consists of both a light and
scanning electron microscopic investigation of eyelid
development in the mouse, covering the period from
eyelid formation, apposition and fusion, through to
their eventual reopening at just under 2 wk after birth.
MATERIALS AND METHODS
Spontaneously cycling female C57BL x CBA F1 hybrid mice were mated with Fl hybrid males and
isolated on the morning offinding a vaginal plug. This
was designated the first day of pregnancy, or 0.5 d
postcoitum (d p.c.). Individual females were killed
by cervical dislocation at various times between 14.5 d
p.c. and full term (approximately 20 d p.c.), and their
embryos isolated into phosphate buffered saline. The
embryos were dissected free of their extraembryonic
membranes and decapitated, and the heads fixed by
immersion in 3 % glutaraldehyde in 0.1 M phosphate
buffer. The heads were then split in the midline in the
sagittal plane. One half of each head was processed
for scanning electron microscopy (SEM), and the
other half embedded in Araldite and sectioned for
viewing by conventional light microscopy.
A selection of newborn mice and weanlings isolated
at various days between birth and 14 d of age were
killed by ether anaesthesia. These were decapitated
and the heads processed for analysis by SEM and light
microscopy. The material that was processed for SEM
analysis was dehydrated, critical point dried, sputter
coated with gold, and then viewed in a JEOL JSM 5200
scanning electron microscope. The material that was
to be analysed histologically was postfixed in 1 %
osmium tetroxide, dehydrated and embedded in
Araldite prior to sectioning at 1 gm. The sections were
stained with 1 % toluidine blue. Between 5 and 8
embryos/weanlings were analysed at each of the
developmental stages studied (i.e. from 14.5 d p.c. to
14 d postnatal).
RESULTS
Although pregnant animals of the same gestational
age were examined, it soon became apparent that the
embryos they contained were at slightly different
stages of development. Consequently in the following
description of eyelid formation, it is the range of
events at a given gestational age that is described.
Gestational age 15.5 days p.c.
A scanning electron micrograph of an embryo at this
stage of eyelid development is shown in Figure 1,
where the cornea of the developing eye can be clearly
seen within the protruding ridges of the future eyelids.
When viewed in the light microscope (Figs 9, 10), the
leading edge of the epithelium is seen to be formed
from a loose aggregation of cells growing out from
each future lid across the corneal surface. The cells at
the leading edges appear to be piled up on the external
surface of the lid. Distant from the growing edge, the
cells of the epidermis consist of a single basal layer of
low columnar cells overlain with 3-4 layers of flattened
cells. Mitosis is common in both basal and adjacent
cell layers. Periderm cells and their associated nuclei
are present on the outer surface of the epidermis. The
epidermis, where it is reflected back from the growing
edge of the eyelid onto its conjunctival surface, is only
2-3 cells thick and therefore much thinner than that
Fig. 2. As development progresses the epidermis (E) of the eyelids appears to be streaming across the corneal surface (C) x 150.
Fig. 3. At higher magnification, the ruffled margins of the eyelids observed in Figure 2 are seen to be formed by a proliferation of peridermal
cells (P) which are overlying the epithelium. x 150.
Fig. 4. Recently fused eyelids showing streaming of the epidermis (E) towards the point of fusion. Also clearly seen is the accumulation of
peridermal cells (P) along the junctional region x 150.
Fig. 5. A high magnification view of the cellular excrescence produced by the peridermal cells (P) seen in Figure 4. x 500.
Fig. 6. This is of an established junctional region late in this period of development. The extent of the cellular proliferation along the line
of fusion is clearly seen (arrowheads) x 150.
Fig. 7. A high magnification of the junctional region shown in Figure 6. P, peridermal cell proliferation. x 500.
Fig. 8. Newborn. By this time the eyelids are tightly fused along the junctional region (arrowheads) and the cellular proliferation, evident
in previous micrographs, is no longer present. x 150.
124
G. S. Findlater, R. D. McDougall and M. H. Kaufman
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Figures 9 to 13 are light micrographs, gestational age 15.5 d p.c.
Fig. 9. The cornea (C) is seen protruding between the developing eyelids. The first signs of the epidermis (E) extending
surface can be seen. D, dermis, L, lens. x 100.
V
across
the corneal
Eyelid closure and reopening in the mouse
on the outer surface. The dermis at this stage of eyelid
development consists of a loose network of cells and
blood vessels.
As development progresses, the epithelium of each
eyelid appears to be streaming across the cornea
towards each other (Figs 2, 3). The epidermis is
thickest where it overlies the dermis at the edge of the
eyelid (Fig. 11). Periderm cells are evident on the outer
surface of each lid and these appear to be continuous
with a less obvious covering of peridermal cells
extending a short distance along the conjunctival
surface of the eyelids. It is now apparent that it is the
periderm which is responsible for the cells which are
loosely situated on the surface of the leading edge of
the outgrowing epithelium.
Figures 4 and 5 are SEM views of recently apposed
eyelids. An analysis of the zone of apposition (Fig.
12), however, reveals that rather than being fused, the
junctional zone between the 2 eyelids is formed from
a narrow band of loosely apposed cells. Adjacent to
the junctional zone, both on the external surface and
to a lesser extent on the conjunctival surface of the
eyelids, peridermal cells appear to be piling up as if
being pushed aside by the coming together of the
epithelium of the 2 eyelids. Keratohyalin granules are
seen for the first time in the outermost layers of the
epidermis distant from the junctional zone.
The dermis of both upper and lower lids appears to
be left behind as rapid proliferation of the overlying
epithelium takes place. The first signs of muscle
formation - representing the future orbicularis oculi are clearly seen scattered throughout the proximal
part of the dermis of both eyelids. The earliest stages
of hair follicle development are also present.
Figures 6 and 7 are from an established junctional
area, and show the extent of proliferation of cells on
the outer surface where the 2 eyelids meet. When a
125
section is taken through this region (Fig. 13), the
cellular excrescence is seen to involve both their outer
and conjunctival surfaces, although it is much less
marked on the conjunctival surface. At this site the
cells in the vicinity of the junction are rounded and
irregularly arranged.
Gestational age 16.5 dp.c.
By this time (Fig. 14) the dermis has extended in
towards the junctional area, leaving a band of
epithelial cells extending from the external surface to
the conjunctival surface. The cellular excrescence on
the external surface is attached by a stalk-like structure
to the line of fusion. The cells pushed out onto the
conjunctival surface are, as before, fewer in number
than on the outer surface. Distant to the area where
the eyelids meet, the epidermis is thrown into
numerous folds. Stratification of the epidermis is now
evident in this location: the basal layer is low
columnar, and superficial to this lie several layers of
paler staining cells. The external epidermal surface
consists of 2-3 rows of flattened cells which have
within them numerous keratohyalin granules. These
granules are not evident in the cellular excrescence
attached to the line of fusion. The epithelium of the
conjunctivum is much thinner than that of the
epidermis, does not appear stratified, and does not
contain keratohyalin granules. Within the dermis
numerous primitive hair follicles are now present,
with the cells of the dermis still concentrated near the
conjunctival surface.
Gestational age 17.5 dp.c.
The most obvious feature at this stage of eyelid
development is the virtual disappearance of the
Fig. 10. A higher magnification view of the epidermis (E) seen in Figure 9 showing that where it passes onto the cornea (C) it consists of
a loose aggregation of cells. x 250.
Fig. 11. This shows the leading edge (arrowhead) of epidermis (E) streaming out across the corneal surface (C). The epidermis is thickest
where it passes out from the underlying dermis (D), L, lens. x 160.
Fig. 12. The junctional region (J) of recently fused eyelids consists of a loose grouping of cells overlain by peridermal cells (P) which appear
to be spilling out onto both the internal and external surfaces. C, cornea. x 350.
Fig. 13. A tight junction now exists between the fused eyelids with no gaps visible between the epidermis (E) of opposing eyelids. The
peridermal cells (P) are seen extending out across the outer surface from the point of eyelid fusion. A smaller accumulation of cells is seen
on the conjunctival surface (arrowheads). x 320.
Fig. 14. Gestational age 16.5 d p.c. The dermis (D) has extended in towards the junctional region (J) producing a thickening of the eyelids.
Periderm cells (P) are still present on both surfaces of the eyelids. x 250.
Fig. 15. Gestational age 17.5 days p.c. Further dermal proliferation in towards the junctional region (J) has produced a narrowing of the
epidermis (E) passing between the eyelids. By this time the cellular excrescence has all but disappeared. x 160.
Fig. 16. Newborn, 20 days p.c. The eyelids are now much thicker. The epidermis can be seen to be differentiated into a basal layer of
columnar cells (arrowheads), 2 or 3 layers of undifferentiated epidermal cells (EC), a stratum granulosum (SG) and several outer layers of
keratin squames (K). On the conjunctival surface, adjacent to the cornea (C), the epidermis (E) is much thinner and nonkeratinised. Immature
hair follicles (HF) are present in the dermis (D) x 160.
126
G. S. Findlater, R. D. McDougall and M. H. Kaufman
cellular excrescence on the external surface overlying
the junctional zone (Fig. 15). The first signs of
keratinisation are apparent by now and are evident in
the region which bridges the 2 eyelids. Keratohyalin
granules are restricted to the most superficial layer of
cells immediately underlying the keratin layers and
cell division is a commonly observed feature in the
basal layer.
The conjunctival epithelium is very much as before,
being thinner than the epidermis. Although the
cellular excrescence on the external surface had
disappeared, a small cellular excrescence on the
conjunctival surface of the fused eyelids was often
seen (Fig. 15). Apart from early evidence of hair
follicle development, which is moderately more advanced in its differentiation than previously observed,
the dermis is similar in appearance to that seen at 16.5
days p.c.
latter. The cells of the stratum granulosum underlying
this depression are larger and rounder than elsewhere.
The epithelium on the conjunctival side of the eyelid
junction consists of round cells with relatively unstained nuclei, unlike the smaller more densely stained
cells of the conjunctival epithelium distant from the
junctional area. The cells of the basal layer are more
darkly staining, particularly in the epithelium of the
junction, and where this passes round onto the
conjunctival surface.
Hair follicles are now numerous in the dermis and
some of these contain hair shafts. Because of the
density of hair follicles within the dermis, the
dermoepidermal junction is greatly folded. Muscle
fibres are also clearly seen to be present between the
hair follicles.
Postnatal d 10
New-born 20 d p.c.
In the new-born mouse (Fig. 16) the junctional
epithelium remains thin relative to the overall thickness of the eyelids. The basal layer of cells on the
epidermal side is columnar in morphology and
remains so as it passes between the 2 lids and onto the
conjunctival surface. On the inner aspect of the eyelids
just distant to the point of union, the conjunctival
epithelium thins from approximately 6-8 cells to
about 3-4 cells in thickness. Where the conjunctivum
thins, the cells also become much flatter in appearance.
On the epidermal surface there are now several
layers of keratin present subjacent to which lies a
distinctive stratum granulosum which occupies about
one-third of the entire thickness of the epidermis. An
SEM of this region (Fig. 8) clearly indicates that the
cellular excrescence previously located along the line
of union of the 2 lids is no longer evident.
Within the dermis, hair follicles are more numerous
and show a greater degree of differentiation than
previously. However, as yet, no hair follicles extend
from the dermis through the epidermis onto the
surface. The first clear signs of tarsal gland development are also present and are located within the
dermis close to the margins of the eyelids.
Postnatal d 5
Figure 17 shows the area of eyelid fusion 5 d after
birth. On the external surface keratin extends without
interruption across the junctional region. However,
there is a noticeable surface depression overlying the
Observation of the eye 10 d after birth reveals that the
eyelids are still closed, with the line of fusion between
them overlain with numerous hairs. An indentation is
also now evident on the conjunctival side of the
junction which corresponds to the depression seen on
the external surface (Fig. 18). Keratinisation is also
seen to be extending in from the epidermal surface
into the region between the 2 eyelids and is also
observed in the region of the indentation on the
conjunctival surface. Numerous mature hair follicles
separated by connective tissue and muscle fibres are
the most characteristic feature of the dermis at this
stage. Tarsal glands are also now clearly seen in the
angle between the eyelid margins and the conjunctival
surface.
Postnatal d 12
By 12 d after birth, eyelid separation is all but
complete, with squames of keratin now present
throughout the junction (Figs 19, 20). At the margins
of the 2 eyelids, the dermis of one eyelid is seen to be
separated from that of the other by a basal layer of
cells which is somewhat more regular in appearance
on the conjunctival surface. Overlying the basal layers
are 2-3 layers of clearer cells, the more superficial of
which contain keratohyalin granules. Keratinisation
is now more extensive on the conjunctival surface
than before, but still only extends for a short distance
along the surface. Thereafter, the conjunctivum
consists of stratified squamous nonkeratinised epithelium. Tarsal glands are now well developed within
the dermis; early evidence of duct formation is also
Eyelid closure and reopening in the mouse
127
Fig 17. 5 d postnatal development. A shallow depression (arrowhead) is visible on the external surface overlying the junctional region (J).
Hair follicles (HF) are visible in the dermis (D). C, cornea. x 160.
Fig. 18. 10 d postnatal development. A corresponding depression is now present (arrowhead) on the conjunctival surface of the fused eyelids.
An extensive stratum granulosum (SG) is present in the junctional region. Mature hair follicles (HF) containing hair shafts are present in
the dermis (D). x 160.
Figs 19 and 20. 12 d postnatal development. Separation of the eyelids is all but complete. Keratin layers (K) now extend downwards between
the 2 eyelids. In Figure 20, the epidermis between the separating eyelids can be seen to consist of stratified squamous keratinised epithelium.
Keratin (arrowheads) extends only for a short distance along the conjunctival surface. Thereafter the conjunctival surface consists of stratified
squamous nonkeratinised epithelium. Tarsal glands (T) are visible adjacent to the free margin of each eyelid. Figure 19, x 100; Figure
20, x 160.
apparent. The latter open at the free margins of the
developing eyelids.
DISCUSSION
The processes involved in eyelid development are
advanced by approximately 2 d in the mouse compared with the rat (Addison & How, 1921). The times at
which these processes occurred in this study corresponded very closely to those reported by Harris &
McLeod (1982). The observation that animals of the
same gestational age were at different stages of
development is also consistent with findings for both
eyelid development in the rat (Addison & How, 1921)
and limb development in the mouse (Maconnachie,
1979) and is a well recognised phenomenon in rodent
development (Theiler, 1989; Kaufman, 1992). However, a distinct progression could still be identified
from early development through to eyelid closure and
subsequent reopening, although the precise timing of
these events could not be determined. Despite this, it
was still possible to gain an appreciation of the rate at
which different processes occurred.
The most striking feature of eyelid development
and closure is the rapidity of events. At the earliest
observed stages of development, seen at about 15.5 d
p.c., the cornea of the eye is clearly visible with the
primitive eyelids represented by protruding ridges of
epithelium at its periphery. By the end of the next
24 h, eyelid formation has progressed to a point where
the cornea of the eye can no longer be seen, being
completely covered by the fused eyelids. The ap-
128
G. S. Findlater, R. D. McDougall and M. H. Kaufman
pearance of the eyelids during this time is of a rapidly
proliferating epithelium with cells of each eyelid
streaming across the corneal surface of the eye
towards each other. Overlying the epithelium forming
the eyelids is a layer of flattened cells termed the
periderm by Bonneville (1968). Peridermal cells are
characteristically found on the epithelial surface of the
fetus, and are continuous with the lining of the
amniotic cavity. At the edges of the gap, the
peridermal cells are seen to be piling up on each other
and produce a substantial cellular excrescence which
overlies the line of apposition of the 2 eyelids.
The initial junction at the site of apposition of the
2 eyelids consists of a loose grouping of cells with
obvious intercellular spaces present between them. As
development continues, and the surface epithelium
becomes stratified, the intercellular spaces disappear
and the junctional zone becomes much more organised. In the human, desmosomes and gap junctions
are found between the epidermal cells in the junctional
zone (Anderson et al. 1967). This observation, if
confirmed in the mouse, indicates that actual fusion
does occur across the junction. On both the external
surface and on the conjunctival surface, epithelium is
seen to be continuous across the junctional zone.
Keratohyalin granules are present and restricted to
the cell layer immediately beneath the periderm but at
this stage are never found on the conjunctival surface.
Once epithelial fusion has taken place, the dermis
continues to develop, causing it to extend in towards
the junctional zone. It is during this period when the
eyelids are fused, which lasts from 15.5 d p.c. to
approximately 5 d of postnatal development, that
eyelid structures start to differentiate. Thus tarsal
glands form during this period, although their
openings into the free margins of the eyelids are not
seen until eyelid separation is almost complete.
The development of hair follicles in the mouse
follows the same pattern as that in the rat (Addison &
How, 1921). The first follicles appear just after eyelid
closure is completed, somewhat distant from the
junctional zone. Further follicle development takes
place towards the junction until, at the start of eyelid
separation, hairs are present on the outer surface of
the eyelid whereas follicles arising in the epithelium of
the junction show only the earliest signs of hair shaft
formation. Addison & How (1921) emphasised the
role of developing hair follicles, especially in the
junctional zone, in the eventual separation of the
eyelids. In the mouse, although hair follicle development is well established in the junctional region,
it appeared to play only an insignificant part in eyelid
At as early as approximately 15.5 d p.c., and before
eyelid fusion occurs, undifferentiated mesenchymal
cells are seen in the area where muscle fibres
subsequently develop. However, by about 16.5 d p.c.,
after eyelid closure has occurred, definitive muscle
fibres are found, though only in the periphery of the
eyelids, distant to the region of eyelid fusion. At
no time, however, is muscle seen adjacent to the
region of eyelid fusion.
The first sign of eyelid separation is the appearance
of a slight depression on the external surface opposite
the epidermal plug which extends between the 2
eyelids. This initial depression appears to result from
a continuing enlargement of the dermis on each side of
the apparently fixed epidermal plug in the junctional
zone. Subsequent deepening of the groove results
from a progressive keratinisation of the epidermal
cells located between the 2 eyelids. Separation of the
2 eyelids by the desquamation of keratin squames
progresses from the epidermal surface at the zone of
apposition towards the conjunctival surface. A corresponding depression is first evident on the conjunctival side in this location at about 10 d after birth.
Keratinisation then appears to extend onto both the
conjunctival and epidermal sides until final separation
occurs at around 12 d after birth.
The processes described here from eyelid development, through eyelid fusion to their subsequent
reopening, are all observed in the human fetus,
although the time course of the entire process is
clearly much more protracted. Sevel (1988) divided
eyelid development into 5 stages: (1) development of
eyelid folds, (2) stage of eyelid fusion, (3) development
of specialised features, (4) eyelid separation and (5)
eyelid maturation. The same stages of eyelid development were observed in the mouse although the
timing of events is measured in days as compared with
weeks in the human. However, bearing in mind this
time difference, there is a close similarity in the relative
duration of each stage of development. The shortest
stage in both the mouse and human was that of eyelid
fold development. The longest stage was from the
time of eyelid fusion through to the time when the
eyelids finally separate. It is during this latter stage in
both the mouse and human that the specialised
structures of the eyelids are developed. Addison &
How (1921) also analysed the development of the
retina and compared the timing of retinal maturation
with eyelid development. They suggested that eyelid
fusion had a protective role protecting all components
of the eye, but particularly the cornea, from exposure
separation.
until the most critical stages of its differentiation
to potentially harmful
substances in the amniotic fluid
are
Eyelid closure and reopening in the mouse
completed. In the mouse, where the stage of development achieved at birth is only equivalent to that
seen at midgestation in the human fetus, this is very
approximately reflected in the timing of eyelid
reopening. It would not be altogether surprising,
therefore, if the degree of differentiation of the cornea
and other intraocular structures reflected the poorer
degree of differentiation of the mouse compared with
the human conceptus at the time of birth.
It is also apparent that eyelid development corresponds closely to the development of epidermal
structures elsewhere in the body. The timing of events
in digit fusion in the mouse (Maconnachie, 1979)
correlates very closely with those of eyelid fusion.
Digit fusion and eyelid fusion both occur at approximately 15 d p.c. and start to separate 3-5 d after birth.
Along the line of fusion a peridermal cell proliferation
occurs which is lost at about 18 d p.c. when the
periderm generally is sloughed off from the underlying
epidermis.
The precise mechanism underlying digit and eyelid
separation is still unclear. However, it has been shown
that a polypeptide, epidermal growth factor (EGF),
enhances both epidermal growth and maturation
(Cohen, 1962). It has also been shown that EGF,
when used in tissue culture medium, can enhance the
lifetime and ability to multiply of human keratinocytes
(Rheinwald & Green, 1977). Green (1977) found that
the rate of squame production and shedding of
epidermal keratinocytes in tissue culture was increased
by 2-5 times in the presence of EGF. When
administered within the first 3 d after birth, EGF, in
the rat, was found to advance eyelid opening by up to
6 d (Birnbaum et al. 1976; Hoath, 1986). The effect of
EGF was reduced the longer the time after birth it was
given, causing Birnbaum et al. (1976) to conclude that
rapidly growing epidermis is more sensitive to EGF
than adult epidermis. It is therefore possible that the
development and growth of the epidermis and its
associated structures result from the rising levels of
EGF in the embryo maximally affecting the rapidly
proliferating epidermis.
Another factor which has been shown to affect the
timing of eyelid opening is the degree ofenvironmental
stimulation to which a newborn rat is subjected.
Smart et al. (1990) showed that the eyelids of rat pups
reared in an 'enriched' environment opened 0.5 d
earlier than those raised in an impoverished environment. To complicate further the issue of eyelid
opening is the observation by Smart et al. (1986) that
the left eye of rat pups reared artificially consistently
opens earlier than the right eye. Why this should be is
unclear. The authors suggested that it may be as a
-
9
129
result of the indirect stimulation that artificially reared
animals receive.
ACKNOWLEDGEMENTS
We thank the Royal College of Surgeons of Edinburgh
and the Scottish Home and Health Department (grant
ref. no. K/MRS/50/C1875) for financial support,
Mr Jack Cable for photographic assistance and
Mr Robert Shields for his help in taking the scanning
electron micrographs.
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